EFFECT OF VERTICAL CONFINEMENT ON DYNAMIC CONE PENETROMETER STRENGTH VALUES IN PAVEMENT AND SUBGRADE EVALUATIONS

The Dynamic Cone Penetrometer (DCP) has become one of the most useful testing devices in pavement evaluation in Israel and other parts of the world. Consequently, a reliable true meaning and interpretation of the results are needed. Research work dedicated to studying the effects of vertical confinement on the DCP strength values of the granular pavement layers and subgrade is summarized. Specifically, four major effects were studied: vertical confinement of granular layers, vertical confinement of cohesive layers, vertical confinement of rigid structural layers, and the effect of vertical confinement of upper asphalt layers on the DCP strength values of the granular layers below them. Based on engineering analysis and experimental testing in the laboratory and in the field, the following conclusions were made. (a) No vertical confinement effect by rigid pavement structure, the upper granular layers, or the upper cohesive layers on the DCP strength values of lower cohesive subgrade layers was found. Any differences in the results between the confined and unconfined DCP values can be explained by the friction that developed in the rod during tilted penetration. (b) However, vertical confinement effects by upper asphalt layers on the DCP values of the granular pavement layers exist. Since this is the true effect of the pavement structure, any DCP measurement for pavement evaluation purposes should be performed through a narrow boring in the asphalt layers and not after removal of a wide strip of asphalt. Generally, these confinement effects decrease the DCP values, and thus increase the structural strength measured. These confinement and friction effects, which can be evaluated quantitatively, should be taken into consideration when using the DCP method to evaluate existing pavements.